Effect of thermophysical properties and injection parameters on microscopic and macroscopic spray characteristics

Fuel spray characteristics are governed by the injection parameters and the fuel's thermophysical properties. The primary objective of this study is to investigate the effects of these parameters on the macroscopic and microscopic spray characteristics, using diesel, ethanol, and methanol as model liquids that span a wide range of viscosities, surface tensions, and latent heats of vaporization. Experiments were conducted at different fuel injection pressures (FIPs) (600, 900, and 1200 bars) and ambient pressures (1, 5, 10, 15, and 20 bars) to study the effect of injection parameters and ambient gas density. Macroscopic and microscopic spray characteristics were investigated using diffused backlit illumination imaging and Phase Doppler Interferometry, respectively. For all test fuels, liquid penetration length (LPL) increases with increasing FIP and decreases with increasing ambient gas pressure. Diesel, having the highest viscosity and surface tension, showed the greatest sensitivity of LPL and spray area (SA) to changes in FIP, as its penetration relied more strongly on initial jet momentum. Methanol, with the lowest viscosity, surface tension, and density among the test fuels, exhibited the shortest LPL and SA, while ethanol showed a longer LPL and higher SA than methanol. Ethanol exhibited the highest arithmetic mean diameter (AMD) and Sauter mean diameter (SMD) among all test fuels across all FIPs, attributable to its lower vapor pressure and higher latent heat of vaporization, which limited the droplet evaporation despite efficient spray atomization. Increasing FIP from 600 to 1200 bars reduced the AMD of diesel and methanol by 16.2% and 17.4%, respectively, whereas ethanol showed negligible size reduction. Methanol exhibited the smallest AMD among all the test fuels due to extensive secondary spray breakup because of its low viscosity and surface tension; however, its SMD (8.42 μm at 1200 bars) remained comparable to baseline diesel (8.17 μm) due to the presence of larger unevaporated droplets alongside the fine droplet population, attributed to high latent heat of vaporization. Methanol showed the highest average droplet velocity, while diesel showed the lowest. Overall, the results establish that the injection parameters and fuels' thermophysical properties dictate macroscopic and microscopic characteristics of the spray.